lundi 1 juin 2015

Gravitational lensing of the cosmic microwave background by galaxy clusters

SPT - South Pole Telescope logo.

June 1, 2015

The photons that make up the cosmic microwave background (CMB) have traversed the universe almost freely for 13.8 billion years, thereby carrying information about the state of the universe when it was only 380,000 years old. "Almost freely" refers to two ways that these photons are disturbed along their long journeys: They are sometimes scattered by hot electrons and they are deflected by deep gravitational wells.

Images above: The left panel shows a simulated map of an unlensed cosmic microwave background. The center panel shows the same map if a large galaxy cluster were along the line of sight. Note that the scale on these two panels goes to 100 microKelvin. The right panel shows the difference between the first two panels. The scale is now down to 10 microKelvin. (Plots are in units of arcminutes.) Images Credits: Antony Lewis and Lindsay King, Institute of Astronomy.

It is this latter deflection, called gravitational lensing, that offers immense promise as a tool to weigh massive objects such as galaxy clusters. Clusters are very important because their abundance offers insight into why the universe is currently accelerating. Extracting this insight, though, requires careful estimates of the masses of clusters. There are currently several techniques in play: X-ray emission, galaxy counts in the clusters, distortions of the shapes of background galaxies and the signal imprinted on the CMB by hot electrons in clusters.

Lensing of the CMB provides a new way to measure cluster masses, one that has just been demonstrated. A simulated signal from one cluster is shown above. Each panel represents about 35 square arcminutes, about 20 times smaller than the moon, so a CMB experiment must have excellent resolution to see the effect. Cluster lensing is the difference between the left and center panels, shown in the right panel. The signal is roughly several microKelvin, much smaller than the typical hot and cold spots that have made the CMB famous. So the resolution must be coupled with exquisite sensitivity.

Image above: The 10-meter South Pole Telescope and the BICEP (Background Imaging of Cosmic Extragalactic Polarization) Telescope at Amundsen-Scott South Pole Station, against the night sky with the Milky Way. Image Credit: NSF.

Large ground-based telescopes such as the 10-meter South Pole Telescope are beginning to attain this dual capability. The noise levels are still too high to measure lensing by a single cluster, so the SPT team performed a likelihood analysis using 513 clusters, detected over three years of the telescope's operation, to measure the weighted mass. The result was a 3-sigma measurement of the lensing of the CMB, with the mass consistent with those obtained with other methods. A paper on this result has recently been accepted for publication in The Astrophysical Journal.

The team is now optimistic that this effect will lead to competitive constraints on cluster masses with upcoming surveys, such as SPT-3G and CMB-S4.